Face recognition is an essential part of social cognition. Despite all faces containing the same general set of features, e.g. eyes, nose, mouth, etc., arranged in the same overall configuration, face recognition is accomplished rapidly and is flexible to ongoing transformation in head orientation, gaze direction, and facial expression. This suggests the presence of a highly specialized cognitive system that is able to process accurately a wide variety of information from faces despite often unique, impoverished, and socially complex viewing situations. Although much is known about the perceptual and neural processes underlying face recognition in humans, very little is understood about the basic behavior and biology of face processing in other species. Those existing studies report conflicting results due, in part, to inconsistent methodologies and the use of poorly standardized stimuli. Therefore, it is unclear whether these cognitive and neural specializations for face processing are unique to humans, or whether they may be present in other socially complex, and cognitively advanced species. Five cohesive experiments are proposed that build on previous studies of social cognition. First, the ability of nonhuman species to process faces based on identity and expression type will be compared using two well-studied experimental methods. In the second and third experiments, the specific perceptual features important for facial expression processing will be compared, along with features important for processing faces based on identity and gender. The fourth experiment will examine whether facial expressions are processed categorically using a morphing procedure similar to that used in studies of human categorical perception. And finally, functional neuroimaging will be used to identify brain regions involved in processing facial expressions and these data will be compared to previous studies of face processing in these two species. Comparative studies of these basic processes in species related to humans are critical for understanding the evolution of social perception as well as for developing research models for studying the neurobiological factors associated with disorders of social cognition. This is important as basic deficits in face recognition and reading and interpreting facial expressions represent common phenotypes for a wide variety of neuropsychological and development disorders in humans, from Alzheimer's disease to autism. PUBLIC HEALTH RELEVANCE: Among humans, there are numerous disorders that include deficits of social cognition, such as face recognition and/or producing and responding to facial expressions. In order to better understand these disorders, nonhuman species are often used as models for the human condition. However, extremely little is known about the basic behavior and biology of socio-cognitive processes in these models. Therefore, it is the goal of these studies to examine the evolution of human face processing skills in order to better understand and treat human diseases.